they can harm roots, but they never visibly harmed my plants when i had em
there are a lot of different treatments
the yellow pest strips catch the adults but don't stop the 6-week life cycle 'cause the eggs are in the soil
don't overwater
a layer of sand (1") on top of your soil will prevent the eggs from maturing
little bowls of vinegar placed around your grow will attract and drown the adult flies

__________________
"We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time." T.S. Eliott